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Hydrologic Properties of Soil and Subsoil on a Steep,. Forested Slope

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Hydrologic Properties of Soil and Subsoil on a Steep,. Forested Slope LI2RARY Hydrologic Properties of Soil and Subsoil on a Steep,. Forested Slope by Darrel Wesley Ranken A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science June 1974 APPROVED: :'%/A4///tt1A Assistant Professor of Forest Engineering in charge of major Head of Departent of ForeEngineering Dean of Graduate School Date thesis is presented March 22, 1974 Typed by Cheryl E. Curb for Darrel Wesley Ranken AN ABSTRACT OF THE THESIS OF Darrel Wesley Ranken for the Master of Science (Name) (Degree) in Forest Engineering presented on (Major) (Date) Title: HYDROLOGIC PROPERTIES OF SOIL AND SUBSOIL ON A STEEP, FORESTED SLOPE Abstract approved: R. DennisHarr The purpose of this study was to examine certain hydrologic properties of the soil and subsoil on a steep forested slope and relate these -rceTties to the movement of water via subsurface routes. The hydrologic properties examined were bulk density, soil texture, total porosity, pore size distribution, saturated hydraulic conductivity, - and soil moisture-tension relationships. Soil samples were taken from a 2. 5 ha study slope on watershed 10 of the H. S. Andrews Experimental Forest near Blue River, Oregon.Eleven soil pits were excavated and six soil core samples were taken at depths of 10, 30, 70, 110, 130,150, and 200 cm where soil conditions permitted.Laboratory analyses were conducted to determine the hydrologic properties of each sample. The extreme permeability and high porosities of the samples necessitated the use of specially designed apparatus to measure the saturated weights and hydraulic conductivities. Particle size distribution changed only slightly with depth.The A and B horizons were predominately clay barns and the C horizons were classified as clays.Total porosities also varied little with depth. The porosity of the soil (A and B horizons) averaged nearly 65% while the porosity of the subsoil (C horizons) averaged nearly 55%.Bulk density also varied little with depth.Soil bulk densities averaged 825 gm/cm3 and subsoil bulk densities averaged 1. 180 gm/cm3. The hydraulic conductivity and pore size distribution of the soil and subsoil were well. correlated and changed considerably with depth. Significant decreases in the hydraulic conductivities occurred between the 30 cm and 70 cm dep-ths as well as between the 110cm and 130 cm depths in some of the soil pits.At most soil pits the surface soil had conductivities greater than 400 cm/hr while the soil at the 70cm and 110 cm depths had conductivities near 200 cm/hr. Subsoils had much lower conductivities, less than 60 cm/hr in most soil pits and less than 10 cm/hr in some pits. A power curve regression analysiswas 'S. used to relate the hydraulic conductivity (Y) and the mean percentage of pores greater than. 294 mmin diameter (X) according to the equa- 2997 Y 10, 040X . The resulting r2 was .945.The percentage of pores greater than . 294 mm in diameter was also found to change abruptly between the 30 cm and 70 cm depths in most soil pits and between the 110 cm and 130 cm depths in some soil pits. The hydrologic properties were used to discuss the possible nature of water movement through the soil and subsoil of the study slope.The soil hydrologic properties and antecedent moisture condi- tions were predicted to be conducive to vertical unsaturated trans- latory flow. A zone of saturation was predicted to occur during winter rainfall events above the subsoil horizon having extremely low con- ductivity rates (above the 130 cm depth near soil pit 1).This zone of saturation was predicted to be the most probable zone of lateral water movement in the form of saturated translatory flow. Data from a soil pit known to have saturated flow over the sub- soil and from tensiometers installed near the soil pits were presented as evideflce that a zone of saturation does exist within the subsoil during some rainfall events and that the soil and subsoil moisture conditions are conducive to translatory flow during the winter rainy season. ACKNOWLEDGEMENTS My gratitude is expressed to the U. S. International Biological Program for the financial assistance that made this study possible. Appreciation is extended to my major professor, R. Dennis Harr, who provided substantial advice, guidance, and assistance throughout the research and preparation of this thesis. Gratitude is expressed to my fellow graduate students for their help and guidance. I also wish to extend special appreciation to my parents and brother, Randy, who have provided the constant faith and encourage-. ment that were necessary for the completion of my graduate work. TABLE OF CONTENTS Page INTRODUCTION 1 LITERATURE REVIEW 4 Energy Conditions Affecting Soil Water Movement 4 Total Potential Energy 4 Darcy's Equation 6 Hydraulic Gradient 6 Hydraulic Conductivity 8 Nature of Soil Water Movement 9 Soil Characteristics Affecting Water Movement 15 DESCRIPTION OF THE STUDY AREA 19 Location 19 Climate 19 Vegetation 22 Geology and Soils MET ECS AND MA TEPJALS- 24 Soil Sampling Methods 24 Soil Pits 24 Soil Sampling 25 Soil Laboratory Analysis Methods 27 Sample Preparation 27 Hydraulic Conductivity 28 Sample Dyeing 31 Saturated Weight 32 Moisture Characteristics 34 Oven Dry Weight 37 Soil Sample Structure 38 Stone Volume 38 Particle Size Distribution 39 RESULTS AND DISCUSSION 41. Soil and Subsoil Characteristics 41 Particle Size Distribution 43 Bulk Density 46 Total Porosity 47 Hydraulic Conductivity 48 Pore Size Distribution 52 Moisture Characteristic 59 TABLE OF CONTENTS (Cont.) Page NATURE OF SOIL WATER MOVEMENT 63 Antecedent Moisture Conditions 63 Unsaturated Flow Through the Soil 65 Saturated Flow in the Subsoil 67 Translatory Flow 67 SUPPORTING EVIDENCE OF PREDICTED SURFACE FLOW 70 Soil Pit 11 Hydrologic Data 70 Tensiometer Data 75 Antecedent Moisture Conditions 76 Nature of Water Flow through the Soil and Subsoil 84 CONCLUSIONS 86 BIBLIOGRAPHY 90 APPENDICES Appendix A 94 Appendix B 100 Appendix C 103 AppendixD 115 LIST OF FIGURES Figure Page 1 Map of study slope. 20 2 Constant head permeameter frame with soil core retainer ring in place. 29 3 C-clamp apparatus. 33 4 Tension table. 35 5 Change in pore size distribution with depth for soil pit 1. 56 6 Hydraulic conductivity vs. pores>. 294 mm in diameter. 58 7 Soil moistu.re characteristic curves for soil pit 1. 60 Soil water tension and rainfall intensity data at tensiometer plot 1. 77 Soil water tension and rainfall intensity data at tensiometer plot 2. 78 10 Soil water tension and rainfall intensity data at tensiometer plot 3. 79 11 Soil moisture content and rainfall intensity data at tensiometer plot 1. 80 12 Soil moisture content and rainfall intensity data at tensiometer plot 2. 81 13 Soil moisture content and rainfall intensity data at tensiometer plot 3. 82 LIST OF TABLES Table Page 1 Mean values of bulk density, particle size distribution, total porosity, and saturated hydraulic conductivity. 44 2 Results of statistical tests comparing mean conductivities between two sampling depths at the 95% level of confidence. 51 3 Mean values of pore size distribution as fractions of total porosity. 54 4 Mean values of hydrologic properties of soil pit 11. 73 LIST OF APPENDIX TABLES BI Mean values of stóne content. 100 BIl Soil moisture, percent of total volume for increasing tension. 101 HYDROLOGIC PROPERTIES OF SOIL AND SUBSOIL ON A STEEP, FORESTED SLOPE INTRODUCTION The rising demand for the Northwest's high qualitywater has increased the necessity for intense management ofour water re- sources. Knowing where the water is and where and how fast it is moving is required for the proper management of this valuablere- source.In the western Cascades of Oregon the prevailing hydrologic process is the subsurface movement of water commonly called inter- flow or subsurface flow. A greater understanding of this important hydro! ph.enomeno would aid water resource management. The combined effects of the Northwest's geologic history, cli- mate, and vegetation have produced a unique soil-water relationship. Long duration, low intensity rainstorms result in the rapid rise of stream hydrographs with the maximum rate of runoff insome instances approaching 80 percent of the average rate of precipitation for the preceding 12 to 24 hours (Rothacher, Dyrness, and Fredrik- sen, 1967).Although streams respond quickly to precipitation vir- tually no overland flow of water has been observedon undisturbed slopes.Nearly all of the precipitation reaching the soil surface passes through the soil and subsoil before entering the open channel drainage system.Considering the percentage of rainfall that has 2 been accounted for in the storm hydrographs, the flashy response of the streams to the onset of winter storm events must be attributed to some form of rapid subsurface flow. The subsurface movement of water also influences other physical phenomena in the forest ecosystem.In addition to contributing to storm flow subsurface water seepage is also responsible for sustained flow of streams during the summer. Subsurface flow is the mechan- ism by which dissolved chemical constituents are removed from the soil and the forest nutrient cycling system (Borman and Likens, 1967). During storm periods in some regions the concentration of water above a relatively impervious layer within the soil profile has resulted in pore water pressures sufficient to reduce total effective weight of a soil mass. This reduction decreases the shear strength of the soil and can trigger mass movements of soil on steep slopes (Swanston, 1970). The precise nature of the mechanism of the subsurface move- ment of water through the soil and subsoil of the western Cascades is not fully understood. Rothacher et al. (1967) have suggested a shallow and rapid lateral movement of water through the soils and sub- soils on the steep slopes of this region.Quantitative information concerning the physical properties of the soil and subsoil is insufficient to describe the nature of this movement.
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